1) Automatic bridge dispenser. Have a pair of cobble generators, with pistons pushing the cobble out. This can be used to make a sort of drawbridge.
2) Concealed Door. This requires sticky pistons, but you can use whatever material everything else is made of, and it's nearly undetectable. For extra sneakiness, don't have a button or pressure plate... you need to be able to drop a redstone torch on a certain square to activate, then pop the torch after you pass through.
3) Floor becomes a staircase. Requires a lot of pistons to go any length, but you can make a secret escape hatch using pistons to depress the floor into a staircase.
4) A redstone signal can go through one solid block. Use a piston as a toggle switch in a redstone circuit?